1. Field of the Invention
The present invention relates generally to methods and apparatus for buffering anesthetics. More particularly, the present invention relates to methods for preparing and storing sodium bicarbonate buffering solutions and combining such solutions with anesthetics stored in small cartridges.
Aqueous solutions containing bicarbonate ions are used in various medical applications such as antidotes, dialysates, artificial cerebrospinal fluid, intraocular irrigating solutions, cardiac perfusates, cardioplegic solutions, peritoneal irrigating solutions, and solutions for organ preservation, etc. Of particular interest to the present application bicarbonates solutions are used to buffer dental and other anesthetics to control pH. One of the most commonly used medical bicarbonate solutions consists of sodium bicarbonate (NaHCO3) mixed with water (H2O). In medical bicarbonate solutions, bicarbonate ions are in equilibrium as represented by the following expression:2HCO3−CO2↑+CO32−+H2O
If the reaction occurs in a closed system, equilibrium is reached with the amounts of reactants remaining constant. In open systems, however, the carbon dioxide gas escapes and the reaction proceeds from the left to the right with bicarbonate (2HCO3) evolving into carbon dioxide gas (CO2), carbonate (CO3) and water (H2O), progressively decreasing the concentration of bicarbonate ions and increasing the concentration of carbonate ions. Since carbonate ions are more alkaline than bicarbonate ions, the pH of the solution will progressively increase.
Clinical effectiveness of bicarbonate medical solutions often depends on maintenance of a particular pH range, generally from 7 to 9. For some applications, maintaining the pH in a more narrow range is beneficial. To stabilize pH and CO2 content, sodium bicarbonate solutions are conventionally packed in gas tight containers that limit leakage of evolved carbon dioxide into the atmosphere. By limiting the loss of evolved CO2 pH change may be reduced. As CO2 leaves solution and enters the container's “headspace” (the gas-filled region above the solution) the partial pressure of the evolved CO2 will increase and eventually establish equilibrium between CO2 leaving solution and CO2 returning to solution.
The gas tight container most commonly used to store medical bicarbonate solutions is the glass vial with a pierceable rubber cap, the cap being referred to as a septum. Such vials allow the medical practitioner to pierce the septum with a hypodermic needle and withdraw or “draw up” a desired volume of bicarbonate solution into a syringe. To facilitate withdrawing the bicarbonate, the vials typically include a significant headspace that prevents a vacuum from forming when the practitioner attempts to draw up the fluid. Once the fluid is drawn up into a syringe, the syringe can be used to deliver the fluid into a catheter or a blood vessel. Of particular interest to the present invention, the partially filled syringe may be used to draw up a second solution, such as a local anesthetic, from another vial in order to mix the second solution with the sodium bicarbonate, where the syringe serves as a mixing and delivery vessel for the resulting pH buffered solution.
One drawback to using such vial-and-syringe systems for storing, mixing, and/or delivering bicarbonate solutions is that drawing up the solution into the syringe reduces pressure over the bicarbonate solution which allows CO2 to leave solution and create CO2 bubbles in the solution during the transfer. Also, there can be significant agitation of the solution as the bubbles enter the syringe, further causing CO2 to dissolve out of solution. For these reasons, even if the pH of a sodium bicarbonate buffering solution in a vial-type storage container were estimated or ascertained before delivery, drawing up, mixing and/or delivery of the bicarbonate system may alter the pH of the solution to an undesirable extent.
One particular device for combining a buffer solution, such as sodium bicarbonate, with an anesthetic, such as a dental anesthetic in a conventional cartridge, is described in U.S. Pat. No. 5,603,695. The device comprises a buffer cartridge having a needle which may be penetrated through the septum of the anesthetic cartridge. The buffer is stored in a cartridge with significant head space and no provision for maintaining volatile CO2 in solution in a bicarbonate anesthetic. Moreover, no provision is made for displacing anesthetic from the anesthetic cartridge as the buffer is introduced.
For these reasons, it would be desirable to provide improved methods and apparatus for combining buffer solutions with anesthetics or other medical solutions, particularly where the buffer solutions are held in conventional glass cartridges. It would be particularly beneficial if the methods and devices employed buffer cartridges which maintained the buffer solution, more particularly sodium bicarbonate solution, in a stable condition with minimal pH change and carbon dioxide loss prior to use. It would be still further desirable if the methods and systems provided for introducing and combining the buffer solutions with anesthetic solution, where the anesthetic solution is held in conventional cartridges, without delivering an excess volume of buffer to the anesthetic cartridge, and relieving or exhausting an equal volume of anesthetic from the cartridge. At least some of these objectives will be met by the inventions described hereinbelow.
2. Description of the Background Art
Glass vials and cartridges for storing medical solutions are described in U.S. Pat. Nos. 1,757,809; 2,484,657; 4,259,956; 5,062,832; 5,137,528; 5,149,320; 5,226,901; 5,330,426; and 6,022,337. Injection pens which employ drug cartridges are described in U.S. Pat. No. 5,984,906. A particular disposable drug cartridge that can find use in the present invention is described in U.S. Pat. No. 5,603,695. A device for delivering a buffering agent into an anesthetic cartridge using a transfer needle is described in U.S. Pat. No. 5,603,695. Devices for maintaining a dissolved gas in solution in a pouch are described in U.S. Pat. Nos. 5,690,215; 5,610,170; and 4,513,015, and U.S. Patent Publ. No. 2007/0265593. Other patents and applications of interest include U.S. Pat. Nos. 2,604,095; 3,993,791; 4,154,820; 4,630,727; 4,654,204; 4,756,838; 4,959,175; 5,296,242; 5,383,324; 5,603,695; 5,609,838; 5,779,357; and U.S. Patent Publ. No. 2004/0175437.